Industrial electrolysis, particularly electrolysis of mainly inorganic acids, is being conducted in an extremely wide range of fields such as electrolytic refining of metals, electroplating, electrolytic syntheses of organic substances and inorganic substances, etc. Although lead or lead alloy electrodes, platinum-plated titanium electrodes, carbon electrodes, and the like have been proposed as electrodes, especially anodes, for use in such electrolytic processes, each of these electrodes has certain drawbacks and, hence, none of them have come into practical use in a wide range of electrolytic applications. For example, lead electrodes, which have on the surface thereof a layer of lead dioxide that is relatively stable and has good electrical conductivity, have drawbacks in that even this lead dioxide dissolves away under conventional electrolytic conditions at a rate of several mg/AH and in that the electrode shows a large overvoltage. A further problem with lead electrodes is that when these electrodes are placed into a cathodically polarized state, the function of the electrodes is impaired because lead metal is more stable than lead dioxide and, hence, the lead dioxide is reduced to lead. Platinum-plated titanium electrodes have a short life for their high price. Further, carbon electrodes have drawbacks in that where the anodic reaction is an oxygen-evolving reaction, the carbon electrodes react with the evolved oxygen and consume themselves as carbon dioxide. Carbon electrodes also have poor electrical conductivity.
Other conventional electrically conductive oxides for use in electrolytic electrodes include manganese dioxide and tin dioxide. However, these two oxides are not being used on an industrial scale because the former oxide has an extremely short anode life and the latter oxide has insufficient electrical conductivity.
In order to avoid these drawbacks of conventional electrodes, a dimensionally stable electrode (DSE) has been proposed and developed and is being used extensively.
The DSE functions as a long-life electrode having exceptionally good chemical stability so long as it employs a valve metal such as titanium as the substrate and is used as an anode, because the surface of the valve metal substrate is passivated. However, when the DSE is used as a cathode and undergoes a cathodic polarization, the substrate is converted into a hydride through reaction with evolved hydrogen and, as a result, the substrate itself becomes brittle or the surface coating peels off due to corrosion of the substrate, leading to a considerably shortened electrode life. This is a serious drawback when the DSE is used in electrolytic processes in which the current flow is reversed.
In addition, the DSE has another problem in that if it is used in an electrolyte solution containing fluorine or fluoride ions in even a slight amount, the valve metal substrate (typically titanium or a titanium alloy) suffers corrosion which shortens the electrode life considerably even when the electrode is used as an anode. For example, if the DSE is used in an electrolyte solution containing fluorine in an amount as slight as from about 3 to 5 ppm, the electrode life is, at the most, one-hundredth the ordinary life of the electrode. Thus, this problem constitutes a serious obstacle to possible applications of the DSE to various electrolytic fields other than soda-producing electrolysis for which the electrode can be used completely satisfactorily.
As a means to overcome the problems described above, it has been proposed to use electrically conductive sintered solids (ceramics) as electrodes. For example, magnetite (Fe.sub.3 O.sub.4), sintered solids having a ferrite magnetite structure, sintered solids having a maghemite structure, and the like are actually being used for electrodes. However, electrodes produced from these materials having a drawback in that although they are relatively stable in neutral or alkaline solutions, the conditions under which they can be used as electrodes in acidic solutions are limited.
In recent years, attention has focused especially on a magneli-phase titanium oxide electrode as an electrode having resistance to fluorine ions. This electrode material is constituted by an electrically conductive titanium oxide which is rather similar to the substances represented by TiO.sub.2-x, so-called suboxides such as Ti.sub.4 O.sub.7. It is known that this titanium oxide in such a stabilized state is never reduced into titanium even when cathodically polarized and suffers almost no corrosion even when anodically polarized. Further, even when the titanium oxide is used in an electrolyte solution containing fluorine ions or a fluoride, it suffers almost no corrosion if the content of such a corrosive substance is 1,000 ppm or less. However, this titanium oxide has slightly insufficient electrical conductivity and, due to this, the quantity of electricity applicable to the titanium oxide is limited. Except for this, the electrically conductive titanium oxide as a material for an electrode or electrode substrate shows attractive properties.
However, since the above-described titanium oxide itself has almost no catalytic activity, the titanium oxide is usually covered with iridium oxide or the like before being used as an electrode. This electrode has a drawback in that the current density limit is low due to insufficient electrical conductivity. Also, when the electrode is used in an acidic solution to conduct an oxygen-evolving reaction on the electrode surface, the titanium oxide of the substrate at the interface with the iridium oxide is converted from Ti.sub.4 O.sub.7 to TiO.sub.2 and is thus passivated and, as a result, application of electric current to the electrode becomes impossible.